In journal articles and research papers, an abstract is a short summary placed prior to the introduction, often with different line justification (blockquote) from the rest of the article, used to help readers determine the purpose of the paper. While the length of the abstract varies by field of study, it is typically a paragraph in length (3-5 sentences), and never more than a page. Similarly, the abstract may or may not have the section title of "abstract" explicitly stated prior to the content. Along with the introduction and conclusion to the paper, the abstract is typically read by those only wishing to gain familiarity with a topic and the results, rather than an in-depth understanding.
An abstract allows one to sift through copious amounts of papers for ones in which the researcher can have more confidence that they will be relevant to his research. Abstracts help a researcher, in other words, decide which papers might be relevant to his research. Once papers are chosen based on the abstract, they must be read carefully to be evaluated for relevance.